Method of making thin metal sections vacuum tight



1958 M. J. ZUNICK ETAL 2,846,753 METHOD'OF MAKING THINMETAL SECTIONSVACUUM TIGHT Original Filed Sept. 28, 1951 '-m 57 MICHAEL J. ZUNIC 52%////A7///4 JOHN E. ILLINGWORTH 48 9M :94 @444 ATTORN EY 'wardly of theenvelope.

2,846,753 Patented Aug. 12, 1958 METHOD OF MAKING THIN METAL SECTIONSVACUUM TIGHT Michael J. Zunick, Greenfield, and John E. Illingworth,

Waukesha, Wis., assignors to General Electric Company, a corporation ofNew York Original application September 28, 1951, Serial No. 248,766.Divided and this application August 18, 1952, Serial No. 304,968

3 Claims. (Cl. 29-4514) The present invention relates in general tovacuum sealing, and has more particular reference to means for andmethod of insuring vacuum tightness at relatively thin sections in metalcastings, the invention more especially pertaining to the assurance ofvacuum tightness in the evacuated envelopes of electron flow devices,such as X-ray generating tubes, more particularly at thin wall sectionsof cast metal comprising portions of the anode of the flow device andforming portions of its evacuated envelope. The invention comprisessubject matter divided from a co-pending application'for United StatesLetters Patent, Serial No. 248,766, filed September 28, 1951.

An X-ray generator tube comprises an electron flow device embodying anelectron emitting cathode, ananode forming an electron target, and asealed and evacuated envelope enclosing the anode and cathode in spacedapart facing relationship within the envelope. X-rays are produced insuch a tube by applying electrical potential between the anode andcathode for electron driving purposes, in order to impel electrons,emitted at the cathode, toward and upon the electron target portions ofthe anode, at relatively high velocity, in order to constitute the anodetarget portions as an X-ray source. The anode and cathode are commonlysealed in openings formed in the envelope,.in fashion disposing portionsof the anode and cathode structure in said openings, therebyconstituting such :portions as parts of the sealed and'evacuatedenvelope. I

impingement of electrons on the'anode target results in the generationofsubstantial quantities of heat, which, especially inhigh poweredtubes, tends to deteriorate and ultimately to destroythe target. In theinterests of efficient X-ray production, the generator tube is usuallyoperated under load conditions producing, in the target, hightemperatures of the order of the melting temperature of the targetmaterial; and various expedients are employed for cooling the anodetarget in order to hold the same at a temperature, during operation ofthe tube, safely below theteinperature at which the target would melt orbecome otherwise damaged or burned.

Water/oil and air are commonly employed as media fortransferring heataway from the anode targetsof X-ray generating tubes. Where air isemployed as a cooling medium, it is common to providev the anodestructure with an extension projecting outwardly of the envelope andcarryingheat radiating fins or other heat dissipating means ,so thatanode heat may be conducted from the target,.through the body of theanode to the-radiating fins, and dissipated thence to circumambientatmosphere out- Such cooling by air is usually employed only inrelatively low power generators, since aircooling is inherentlyincapable of dissipating heat at a sufficiently high rate to accommodatethe relatively large quantities of anode heat generated in high powertubes when in operation. Accordingly, for high power tube coolingpurposes, it is common to continuously deliver a cooling medium, such asoil or water, through a chamber formed in the anode structureimmediately behind the target, the medium being circulated between thechamber and an external cooler to thus absorb heat from the target intothe circulating medium and then extract the heat from the mediumoutwardly of the generator and dissipate it to atmosphere. The mediumthus circulated in the chamber formed in the anode structure,immediately behind the target, is normally under pressure of the orderof atmospheric pressure, as distinguished from the low pressurecondition-s within the evacuated envelope, so that the sectionalthickness of the target portions of the anode "structure, at saidchamber, are required to be vacuum tight in order to preserve theevacuated condition within the generator envelope.

It is desirable, however, to make the said target portions as thin aspossible, in the interests of rapid heat dissipation from the anodetarget to the circulating medium in the chamber. Anode structures,however, are commonly made of cast metal, usually copper, the targetusually comprising a button of refractory material, such as tungsten,embedded in the copper anode structure adjacent the cooling chamber. Ithas been found necessary, in the past, to design X-ray tube anodes withwalls of thicknesses of the order of not less than of cast metal, behindthe target button and between it and the cooling chamber, because of therelatively coarse granular structure obtained during the castingoperation, which tends to open up along the grain-boundaries of theconstituent material during subsequent heat processing of the casting.

An important object of the present invention is to substantially reducethe sectional thickness in metal castings, as between the target buttonand the cooling chamber in an X-ray anode structure, while maintainingadequate vacuum tightness at such section of reduced thickness; afurther object being to provide an X-ray anode structure embodyingminimal wall thickness immediately behind the target button, in theinterests of rapid heat dissipation.

Another important object is to accomplish minimal sectional thickness incast metal structures, while maintaining vacuum tightness therein, byincorporating, in the structure, a relatively thin layer or sheet ofrelatively dense material, such as molybdenum, as a vacuum maintaininginsert in the structure at the place or station where vacuum tightnessis desired.

Another important object is to incorporate, in a casting comprising athin cast metal section, a thin plate or shell of dense, substantiallyimpervious material, in order to render the thin cast metal sectionvacuum tight; a further object being to incorporate such shell or layerin the casting during the formation thereof as such, by applying theshell or plate as an insert in the casting mold and casting the metalinto the mold in position to envelop and embed the insert in the castmetal.

Another important object is to provide an X-ray tube anode comprising ametal casting carrying a target button embedded therein at a relativelythin wall portion which defines a coolant circulating chamber in thestructure, the casting incorporating a thin layer of molybdenumenveloped in the casting between the button and the cooling chamber.

The foregoing and numerous other objects, advantages and inherentfunctions of the invention will become apparent as the same is morefully understood from the following description which, taken inconjunction with the accompanying drawings, discloses preferredembodiments of the invention for the purpose of demonstrating the same.

Referring to the drawings:

Fig. 1 .is a sectional view of an X-nay generating tube having an anodestructure of cast metal, having a thin 3 Wall section containing avacuum maintaining insert in accordance with the present invention;

Fig. 2 is an enlarged sectional view taken through the anode structureshown in Fig. 1;

Fig. 3 is a sectional view taken substantially along the line 33 in Fig.2;

Fig. 4 is a sectional view taken through another anode structureembodying the invention;

Fig. 5 is a sectional view taken substantially along the line 5-5 inFig. 4;

Fig. 6 is an exploded view illustrating the manner of mounting thepre-formed target button and the vacuum sealing shell or layer asinserts in an anode casting mold; and

Fig. 7 is a sectional view showing another anode structure embodying theinvention.

To illustrate the invention, the drawings show an X-ray generating tube11 comprising a cathode 12 embodying an electron emitting element 13, ananode 14 forming an electron target 15, and a sealed evacuated envelopeenclosing and supporting the anode and cathode in spaced apart facingrelationship within the tube. While the invention is not necessarilylimited to any particular form, construction or configuration of theenvelope, or of the cathode 12, the envelope, as shown, may comprise atubular glass element 16 having reentrant end portions 17 and 17,forming openings at the opposite ends of the envelope. These endportions may be circumferentially sealed respectively to the anode andcathode structures, in order to hermetically seal the envelope at saidopenings and to mechanically support the anode and cathode structuresrespectively upon the reentrant portions 17 and 17, so that the anodeand cathode structures seal said openings and form parts of the envelopestructure within the reentrant portions 17 and 17.

The emitting element 13 may comprise a filament suitably supported onand insulated from the cathode structure 12, the filament beingconnected with lead conductors 18 adapted for connection with a suitablepower source outwardly of the envelope, for the purpose of energizingthe filament for electron emission. The filament may be connected withthe lead conductors 18 through suitable envelope seals, preferablyformed in the envelope forming parts of the cathode structure, at theinner end of the reentrant portion 17.

It will, of course, be understood that the generator tube 11 may beoperated for the production of X-rays at the target 15, by energizingthe filament 13 for electron mission, as by connecting the conductors 18with a suitable source of emitter energizing power, while simultaneouslyapplying electron driving potential between the cathode and the anodetarget, as by connecting the anode and one of the conductors 18 with asuitable source of electron driving potential outwardly of the envelope.To this end, the necessary electrical connection with the anode may bemade at or through the outwardly exposed envelope forming parts thereofdisposed within the reentrant portion 17.

Electrons emitted by the filament 13, when energized, may travel as anelectron beam 19 from the filament 13 under the influence of theanode-cathode electron driving potential, and impinge upon the facingsurface of the target 15, thereby constituting the target as a source ofXrays which may be projected thence as an X-ray beam 20.

The present invention is not necessarily limited to any particular anodeshape, style or configuration. The :anode may comprise a body 21 formedof cast metal, such as copper, into which is set a target button 22 ofsuitable refractory target material, such as tungsten. Behind the targetbutton 22, the body 21 may form a wall 21 defining a cavity 23 for thecirculation of a cooling fluid in heat exchange relationship with thebutton 22, through the wall 21', for the continuous cooling of thetarget, the sectional thickness of the cast metal wall 21 behind thebutton 22 being of the order of or less, which is ordinarilyinsuflicient to provide vacuum tightness in a cast metal wall of suchthickness. In order to insure vacuum tightness in said relatively thinsection of cast metal behind the button 22, a layer 24 of dense,impervious sheet material is embedded in the Wall 21. As shown moreparticularly in Figs. 1, 2 and 3, the layer 24 may comprise a cup-shapedelement having a bottom wall 25 extending in the wall 21' immediatelybehind the target button 22, the vertical spacement between the facingsurfaces of the button 22 and the wall 25 being of the order of or less.

The member 24 preferably comprises molybdenum and has wall thickness ofthe order of from 0.005" to 0.010" so that the combined thickness ofcopper, molybdenum, and the material of the target 21, between thecathode facing surface of the target button and the chamber 23, is ofthe order of A5", or less, of which distance the thickness of the targetbutton accounts for about The anode structure, also, if desired, may beformed with staggered partition forming fins 26 and 26' extending in thechamber 23. These fins may be integral with the side and bottom walls ofthe cavity 23 and may include a pair of spaced apart lateral fins 26extending from one side of the cavity toward the opposite side thereof,as shown more particularly in Fig. 3 of the drawings, and a central fin26' spaced from and extending medially between said lateral fins fromthe opposite side of the cavity 23. These fins may serve to strengthenthe wall structure, and will also serve as deflecting baffles forcooling fluid circulated in the chamber 23, as hereinafter more fullydescribed.

The anode structure may also include a sleeve-like extension 27, one endof which may be brazed or otherwise sealingly secured on the casting 21,as at 28, in open communication with the cavity 23. The opposite or openend of the member 27 may be fitted with a cover member 29 carrying theouter end of a tube or pipe 30 in position extending outwardly of thecover member 29 for connection with a source or cooling fluid. The innerend of the pipe 30 may be sealed, as at 31, in an opening 32 formed in apreferably metal disk 33.

This disk may be sized to extend snugly within the cavity 23 in positionoverlying the wall remote edges of the fins 26 and 26, the opening 32being disposed opposite the wall remote end of the fin 26'. Outwardly ofthe fins 26, the disk 33 may be formed with a pair of cut-out openings34; and the end closure member 29 may be formed with an outlet opening35. A suitable cooling fluid may be delivered, as by means of a pump orother circulating device, through the pipe 30 from a source of suchfluid disposed outwardly of the X-ray generator tube. The cooling fluidmay pass thence through the opening 32 into the cooling chamber 23. Suchfluid may travel in the chamber 23 through the channels defined by andbetween the baffles 26 and 26' in heat exchange relationship with thetarget button through its supporting wall portions of the anode. Thecooling fluid may be discharged from the chamber 23 through the openings34 into the sleeve-like member 27, and may pass thence through theoutlet opening 35 which, if desired, may be connected with the externalsource of cooling fluid, preferably through suitable means for coolingthe fluid for recirculation to the chamber 23.

In order to seal the anode in the envelope member 16, the body 21 of theanode may be formed with a pcripheral shoulder 36 upon which is sealed,as by welding or brazing, a cup-shaped seal member 37 having annularskirt portions forming a lip adapted to form a circumferentialglass-to-metal seal 38 with the reentrant envelope portion 17, at theinner end thereof. The anode may thus be sealingly mounted and supportedon the envelope member 16 so that the seal member 37, as well asportions of the anode including the target supporting wall portions 21',may form portions of the envelope structure closing the opening formedat the inner end of the reentrant portion 17. V

The present invention, of course, is not necessarily limited to the formand arrangement of the parts of the anode structure. As shown moreparticularly in Figs. 4 and 5 of the drawings, the body 21 may be formedwith asingle medial partition wall 26" formedintegral with the opposite.side walls of the chamber 23 and dividing it into a pair ofcompartments 23' on opposite sides of the partition, said partition,wall having an edge spaced from the bottom 25 of the cup-shaped memberto define a slot or opening 23". between said edges and the bottom ofthe cup. This slot serves to interconnect the compartments 23 and toassure that the cooling medium, in flowing in the chamber fromonecompartment to the other, will pass immediately behind the target, inefficient heat exchange relation therewith.

The disk 33, which covers the open side of the chamber 23, in Figs; 4and 5, may be formed with inlet and outlet openings 32 and 34' disposedtherein in position respectively communicating with the compartments 23on opposite sides of the walls 26", the inlet pipe 30 being sealed inthe opening 32, at the inner end of said pipe. The disk 33 also may besealed at its edges to the inner end of the sleeve-like extension 27,which, in turn, may be sealed in the body 21 as at 2 8, and a sealingskirt 37, making a glass-to-metal seal 38 with a reentrant glassenvelope portion 17, may be sealed, as at 36, upon peripheral shoulderportions of the body 21.

An enclosing, preferably sheet metal, shell 40 may be applied upon thebody 21 and secured in position, as by crimping the peripheral portions41 of the shell into locking engagement in circumferential grooves 42formed in the body 21. The shell 40 may include skirt portions 43extending around and enclosing the glassmetal seal 38 to protect thesame from stray electron impingement. The shell may also be formed'withportions 44 enclosing the target end of the anode, such portions beingformed with an opening 45 in alinement with and between the electronemitting element of the cathode and the anode target formed by thebutton 22.

It is, of course, also possible to incorporate structures embodying thepresent invention in other types of anodes, including anodes for use inX-ray generators of the sort shown in United States Letters Patent No.2,356,645, issued August 22, 1944, on the invention of Z. J. Atlee andH. W. Brackney. Such an anode is shown in section, in Fig. 7, and maycomprise a tubular metal sleeve 46, forming a portion of the envelope ofthe generator tube, a target structure 47 being sealed in the end of thetube in position to be bombarded by electrons emitted by a cathodeenclosed in and supported by the tube envelope at the target remote endof the tube 46. The cathode is thus in position to emit an electron beamlongitudinally through the tube 46 for impingement upon the target. Asshown in Fig. 7, the target structure 47 may comprise a body 21 of castmetal, such as copper, in which a target button 22 of refractorymaterial, such as tungsten, is set, the structure including a disk orplate 25 of dense, impervious material, such as molybdenum, enveloped inthe copper body of the target structure behind the target button 22, thebody 21 including an outwardly extending peripheral flange 48 tofacilitate the sealing of the target structure 47 in the end of the tube46, as by welding or brazing the tube end to said flange.

Anode structures embodying the present invention, regardless of theirshape or configuration, may be formed by placing the target button 22and the member 25, Whether formed as a cup or as a plate, as inserts ina suitable casting mold in which the inserts may be anchored. To thisend, as shown more particularly in Fig. 6 of the drawings, the targetbutton 22 may be anchored on a mounting plate 49, preferably comprisinggraphite, by means of molybdenum anchoring pins 50, said pins 6havingends secured in openings 51 in the anchorplate. The-member 25,whether the same comprises a flat plate as shown in Fig. 7, or thebottom of a cup-shaped member of the sort shown in Figs. 3 and 4, may beanchored on the mounting plate 49 in stacked relationship with respectto the button 22, as by means of molybdenum anthe member 25 is supportedin spaced parallel relationship above the target button 22, the verticaldistance between the facing surfaces of the member 25 and the button 22being equal to the diameter ofrthe hooked disk retaining ends of thepins 50. The inserts 22 and 25, thusanchored on the mounting plate 49,may be fastened in a suitable mold shaped in conformity with the desiredshape of the anode body 21, and copper may then be cast into the mold,upon the so mounted insert elements 22 and 25 in order to embed andenvelop said elements in the casting in proper relative positiontherein, the button 22, in effect, being brazed to and upon the member25, by the intervening cast material which enters between the spacedefined by the hooked portions of the anchor pins 50, between the facingsurfaces of the elements 22 and 25. The casting operation is preferablyaccomplished under vacuum conditions. After the casting operation hasbeen completed and the anode structure removed from the mold, themounting disk 49 may be stripped from the casting and the projectingends of the pins 50 and 52 may be removed, leaving portions of theanchoring members 50 and 52 embedded in the cast material in the edgesof the target button.

It will be seen from the foregoing that the present invention provides anovel method of insuring vacuum tightness in the metal castingcomprising the thin wall section 21' which extends behind the targetbutton 22 and between it and the chamber 23, such wall sectioncomprising a part of the evacuated envelope of the X-ray tube. This isaccomplished by embedding in said section a relatively thin layer ofdense, impervious material, such as molybdenum. The resulting structureis not only vacuum tight at said thin wall section, but said wallsection,

being thin, allows for the rapid transfer of heat from the target button22 to the cooling fluid in the chamber 23.

X-ray anodes incorporating the novel features of the present inventionnot only show improved and efficient anode cooling characteristics; butthe incorporation of integral coolant guiding baffles, such as thebaffies 26 and 26, in the anode structure eliminates the necessity ofassembling relatively more expensive conventional cooling coils in thechamber 23, the guiding baffles 26 and 26'. together with theinexpensive cover disk 33, being a cost reducing improvement in fluidcooled X-ray tube anodes.

It is thought that the invention and its numerous attendant advantageswill be fully understood from the foregoing description, and it isobvious that numerous changes may be made in the form, construction andarrangement of the several parts without departing from the spirit orscope ofthe invention, or sacrificing any of its attendant advantages,the forms herein being preferred embodiments for the purpose ofillustrating the invention.

The invention is hereby claimed as follows:

1. The method of making vacuum tight an X-ray tube anode of cast metalhaving a target button mounting wall of relatively thin sectionaldimension, which comprises pre-forming, as separate elements, the targetbutton and a vacuum sealing member comprising a thin layer of denseimpervious metal, mounting said button and sealing member in closelyspaced, surface facing relation as inserts in a mold, and then casting asuitable anode metal into the mold under vacuum conditions to envelopand embed said sealing insert in the cast metal comprising said mountingwall, between the opposite faces thereof,

and to apply a joining layer of the cast metal between the closelyspaced facing surfaces of said inserts to integrate the same by and withsaid joining layer.

2. The method of rendering a thin cast metal wall section vacuum tight,whereby the same may be employed for rapid heat transfer therethroughwhile exposed to substantial pressure differentials on opposite sides ofsaid section, which comprises mounting a layer of dense, impervious,heat conductive metal forming a vacuum sealing shell as an insert in acasting mold, and then casting a suitable wall forming metal, capable ofbrazingly joining with the material of said shell, into the mold inposition to envelop and embed said sealing shell in the cast metalforming said wall section, between the opposite faces thereof.

3. The method of making vacuum tight an X-ray tube anode of cast copper,having a target button mounting wall of relatively thin sectionaldimension, which comprises preforming, as separate elements, the targetbutton and a vacuum sealing member comprising a thin layer of dense,impervious molybdenum, mounting said button and sealing member inclosely spaced, surfacefacing relation as inserts in a casting mold, andthen casting copper into the mold under vacuum conditions, to envelopsaid sealing member in said copper, between the opposite faces of saidmounting wall, and to embed said button in the copper, at a face of saidwall, while applying a joining layer of copper between the closelyspaced facing surfaces of said inserts to integrate the same by and withsaid joining layer.

References Cited in the file of this patent UNITED STATES PATENTS

